<template>
  <ul class="list" >
    <li class='item' v-for='(item,index) in letters' 
      :ref='item'
      :key='index' @click='handleLetterClick'
      @touchstart.prevent='handleTouchStart'
      @touchmove='handleTouchMove'
      @touchend='handleTouchEnd'>
      {{item}}
    </li>
  </ul>
</template>

<script>
export default {
  name: "CityAlphabet",
  props: ["cities"],
  data() {
    return {
      touchStatus: false,
      startY: 0,
      timer: null //节流限制函数执行的频率
    };
  },
  computed: {
    letters() {
      const letters = [];
      for (let i in this.cities) {
        letters.push(i);
      }
      return letters;
    }
  },
  updated() {
    this.startY = this.$refs["A"][0].offsetTop;
  },
  methods: {
    handleLetterClick(e) {
      this.$emit("change", e.target.innerText);
    },
    handleTouchStart() {
      this.touchStatus = true;
    },
    handleTouchMove(e) {
      if (this.touchStatus) {
        if (this.timer) {
          clearTimeout(this.timer);
        }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY - 79;
          const index = Math.floor((touchY - this.startY) / 20);
          if (index >= 0 && index < this.letters.length) {
            this.$emit("change", this.letters[index]);
          }
        }, 16);
      }
    },
    handleTouchEnd() {
      this.touchStatus = false;
    }
  },
  components: {}
};
</script>

<style lang='stylus' scoped>
@import '~styles/variables.styl'
.list
  display flex
  flex-direction column
  justify-content center
  position absolute
  top 1.58rem
  right 0
  bottom 0
  width 0.4rem
  .item
    line-height 0.4rem
    text-align center
    color $bgColor
</style>
